1. Field of the Invention
The present invention generally relates to the art of manufacturing electrical components. In particular, the present invention relates to the art of fabricating capacitors.
2. Description of Related Art
The conventional method for fabricating capacitors on semiconductors using analog fabrication process technology is relatively simple because the analog fabrication process involves the use of two polysilicon layers. In the analog process, as illustrated by FIG. 1, the capacitors are fabricated using a two polysilicon layers. In FIG. 1, the first layer 12 of polysilicon forms the "Bottom Plate" of the capacitor while the second layer 10 of polysilicon forms the "Top Plate" of the capacitor 8. In addition, because the capacitance per unit area of the polysilicon-polysilicon type of capacitors is relatively large, the physical area required by such capacitors is relatively small.
Fabrication of capacitors on semiconductors is difficult when the digital circuit fabrication process technology is used because the digital process does not utilize multiple polysilicon layers. Therefore, the conventional method of fabricating capacitors for a digital process involves the use of metal layers as the plates of the capacitors. As illustrated by FIG. 2, a conventional capacitor in a digital process comprises a metal top plate 14, sandwiched between two metal bottom plates 16 and 18.
Because the capacitance per unit area of the metal-metal type of capacitors is relatively small, the physical area required by such capacitors is relatively large. In an effort to reduce the physical area occupied by the metal-metal capacitors, the capacitor 20 illustrated by FIG. 2 has the top plate 14 being sandwiched between two bottom plates 16 and 18 which are connected 17 to each other. This structure doubles the surface area between the top plate 14 and the bottom plates 16 and 18 compared to the surface area of capacitors having the more typical structure of one top plate and one bottom plate.
However, even with the use of the sandwich technique, the metal-metal capacitors occupy larger physical areas compared to the polysilicon-polysilicon capacitors of the same capacitance. Due to their larger size, the metal-metal capacitors introduce greater levels of noise to the substrate 22 compared to the polysilicon-polysilicon capacitors, and also are more easily affected by the substrate noise.